Keywords: concrete temperature stress crack control
A, concrete cracks
According to the depth, concrete cracks can be divided into three types: through cracks, deep cracks and surface cracks. Through cracks develop from concrete surface cracks to deep cracks, and finally form through cracks. It cuts off the cross section of the structure, which may destroy the integrity and stability of the structure, and its harm is more serious; However, it is also harmful for deep cracks to cut off the structural section; Surface cracks are generally less harmful. However, the appearance of cracks does not absolutely affect the structural safety, and it has a maximum allowable value. Under normal indoor environment, the maximum crack width of general members is less than 0.3mm；; The maximum crack width of components in outdoor or indoor high humidity environment is less than 0.2 mm.
For underground or semi-underground structures, concrete cracks mainly affect its waterproof performance. Generally, when the crack width is 0. 1-0.2mm, although there is slight water seepage in the early stage, after a period of time, the crack can heal itself. If it exceeds 0.2-0.3mm, with the increase of crack width, the leakage water will increase rapidly. Therefore, underground engineering should try to avoid cracks with a total section of more than 0.3 mm. If this kind of crack occurs, it will greatly affect the use of the structure, and chemical grouting must be carried out to strengthen it.
Temperature cracks will also occur in the construction stage of mass concrete. On the one hand, it is the internal factors of concrete: due to the temperature difference between inside and outside; On the other hand, it is the external factors of concrete: the external constraints of structure and the constraints between concrete particles prevent the shrinkage and deformation of concrete. The compressive strength of concrete is large, but the tensile force is small, so once the temperature stress exceeds the tensile strength that concrete can bear, cracks will appear. The width of this crack is within the allowable range, which generally does not affect the strength of the structure, but it does affect the durability of the structure, which must be paid attention to and controlled.
Second, the analysis of the main causes of cracks
1. Hydration heat of cement
Cement will release a certain amount of heat in the hydration process, especially the mass concrete structure has a thick cross section and a relatively small surface area coefficient, so the heat generated by cement is not easy to be lost when it gathers inside the structure. In this way, the hydration heat inside the concrete can not be released in time, so that the more it accumulates, the greater the temperature difference between inside and outside. The hydration heat of cement released by concrete per unit time is related to the amount and variety of cement per unit volume of concrete, and increases with the age of concrete. Because the surface of concrete structure can naturally dissipate heat, in fact, the highest internal temperature mostly occurs in the first 3-5 days after pouring.
2. Changes in external temperature
In the concrete construction stage, its pouring temperature changes with the change of external temperature. In particular, the sudden drop in temperature will greatly increase the temperature difference between the inside and outside of concrete, which is extremely unfavorable to concrete. Temperature stress is caused by temperature deformation caused by temperature difference; The greater the temperature difference, the greater the temperature stress. At the same time, under the condition of high temperature, concrete is not easy to dissipate heat, and the highest temperature in concrete can generally reach 60-65℃ and last for a long time. Therefore, temperature control measures should be taken to prevent temperature stress caused by temperature difference between inside and outside of concrete.
3. Concrete shrinkage
About 20% water in concrete is necessary for cement hardening, while about 80% water will evaporate. The evaporation of excess water will cause the shrinkage of concrete volume. The main reason of concrete shrinkage is that internal water evaporation leads to concrete shrinkage. If concrete is saturated with water after shrinkage, it can resume expansion and almost reach its original volume. Dry-wet alternation will cause the alternating change of concrete volume, which is very unfavorable to concrete. The shrinkage of concrete is mainly influenced by cement varieties, concrete mixture ratio, admixture and admixture varieties, construction technology (especially curing conditions).
Third, the concrete material requirements
The raw materials selected for concrete should pay attention to the following points:
1. Coarse aggregate shall be graded continuously, and fine aggregate shall be medium sand. Additives should be retarder and water reducer; Admixtures should be fly ash and slag powder. On the premise of ensuring the strength and slump of concrete, the content of admixture and aggregate should be increased to reduce the cement consumption of unilateral concrete.
2. Cement with low hydration heat and long setting time should be selected as far as possible, as well as medium heat portland cement, low heat slag portland cement, dam cement, slag portland cement, fly ash portland cement and pozzolanic portland cement. Is the first choice. However, slag cement with low hydration heat has more water precipitation than other cements, and a large amount of water is precipitated on the surface of pouring layer. This bleeding phenomenon not only affects the construction speed, but also affects the construction quality. The change of water-cement ratio of concrete is because the separated water gathers between the upper and lower pouring layers, and the excavated water takes away part of mortar, forming an interlayer with high water content, which destroys the cohesion and integrity of concrete. Concrete bleeding is related to water consumption, which is large. And it is related to temperature. With the increase of temperature, the time for complete precipitation of water is shortened. In addition, it is also related to the composition and fineness of cement. Therefore, when choosing slag cement, we should try our best to choose the variety of bleeding water, and add water reducer to concrete to reduce water consumption. During construction, the separated water should be discharged in time or a certain amount of hard concrete should be mixed at the separated water and poured evenly, and then a layer of concrete should be poured after vibrating with a vibrator. Fourth, concrete pouring
Besides ensuring that each layer of concrete is covered with a new layer of concrete before initial setting and tamping, the pouring scheme should also consider the influence of structural size, steel bar density, design of embedded pipes and anchor bolts, concrete supply, hydration heat and other factors. Commonly used methods are as follows:
1. Fully layered
That is, after the first layer is poured, pour the second layer. At this time, the first layer of concrete has not been initially set, so it will be continuously poured step by step until it is completed. Using this scheme, the plane size suitable for the structure should generally not be too large, and it is more suitable to start from the short side and advance along the long side during construction. If necessary, it can be divided into two sections, from the middle to both ends or from both ends to the middle.
2. Segmentation and stratification
When pouring concrete, start from the bottom, pour to a certain distance before pouring the second layer, and then pour other layers forward in turn. Due to the large number of total floors, after pouring to the top, the concrete at the end of the first floor has not yet been initially set, so it can be poured in layers from the second paragraph. Different from the first scheme, this scheme is suitable for the case that less concrete needs to be supplied per unit time. This scheme is suitable for projects with large area or length but not too large structural thickness.
3. Slope stratification
It is required that the slope of the inclined plane is not more than 1/3, which is suitable for the case that the length of the structure greatly exceeds 3 times the thickness. The concrete starts from the lower end of the pouring layer and gradually moves upward.
Five, the temperature control of concrete curing
The curing of concrete should not only meet the needs of strength growth, but also prevent concrete cracking caused by temperature deformation by artificially controlling temperature. Temperature control is to artificially control the pouring temperature of concrete and the highest temperature inside concrete. Temperature control in concrete curing stage should follow the following two points:
1. When concrete is dismantled, the temperature difference of concrete shall not exceed 20℃. The temperature difference shall include the temperature difference between the surface temperature, the center temperature and the outside air temperature.
2. Insulation method is to cover the exposed concrete surface of the structure and the outside of the formwork with insulation materials (such as straw bags, sawn wood, wet sand, etc.). ), make the concrete in the process of slow heat dissipation to obtain the necessary strength, in order to control the temperature difference between inside and outside the concrete is less than 20℃.
References:
[1] average. Research and application of low-cost high-performance concrete [D]. Wuhan University of Technology, 2004.
[2] Wang Jiachun, Yan. Influencing factors of adiabatic temperature rise of concrete [J]. Concrete and cement products, 2005, (3) The main causes of concrete cracks are temperature and humidity, and concrete is often affected by external temperature changes during construction. The internal temperature of condensation is the superposition of adiabatic temperature and pouring temperature of cement hydration heat, in which pouring temperature is directly related to external temperature. Generally speaking, the higher the external temperature, the more hydration heat released by cement during the hardening of concrete, and the higher the pouring temperature of concrete. When the temperature drops, the temperature gradient inside the outer concrete will greatly increase, resulting in temperature difference and temperature stress, and cracks will appear in mass concrete.
In order to prevent cracks and reduce temperature stress, we can control the temperature and improve the constraint conditions.
Measures to control the temperature are as follows:
(1) Take measures such as improving aggregate gradation, using hard concrete, mixing mixture, adding air entraining agent or plasticizer to reduce the cement content in concrete;
(2) When mixing concrete, add water or water to cool the crushed stone to reduce the pouring temperature of concrete;
(3) When pouring concrete in hot weather, reduce the pouring thickness and use the pouring layer to dissipate heat;
(4) embedding water pipes in concrete and introducing cold water for cooling;
(5) Set a reasonable time for formwork removal, and conduct surface insulation when the temperature drops suddenly to avoid a sharp temperature gradient on the concrete surface;
(6) During construction in cold season, thermal insulation measures should be taken for the surface of concrete pouring blocks or thin-walled structures exposed for a long time;
Measures to improve the constraint conditions are:
(1) Reasonable split and block;
② Avoid excessive foundation fluctuation;
(3) Reasonable arrangement of construction procedures to avoid excessive height difference and long-term side exposure;
In addition, it is very important to improve the performance of concrete, improve crack resistance, strengthen maintenance, prevent surface drying shrinkage, and especially ensure the quality of concrete. Special attention should be paid to avoid through cracks, which are difficult to restore structural integrity. Therefore, priority should be given to preventing through cracks in construction.
In concrete construction, in order to improve the turnover rate of formwork, it is often required that the newly poured concrete be removed as soon as possible. When the concrete temperature is higher than the air temperature, the time of formwork removal should be properly considered to avoid early cracks on the concrete surface. Dismantling the formwork in the early stage of new pouring causes great surface tensile stress and "temperature shock" phenomenon. At the early stage of concrete pouring, due to the effect of hydration heat, considerable tensile stress is generated on the surface, and the surface temperature is also higher than the air temperature. At this time, when the mold is removed, the surface temperature drops suddenly, which will inevitably cause a temperature gradient, which will increase the surface tensile stress, which will be superimposed with the hydration thermal stress, and the surface tensile stress will reach a large value, and cracks will occur. However, if the surface is covered with light thermal insulation materials, such as foam sponge, in time after formwork removal.
In order to ensure the quality of concrete engineering, prevent cracking and improve the durability of concrete, the correct use of additives is also one of the measures to reduce cracking. For example, the use of water reducing agent and anti-cracking agent, its main role is:
(1) There are a lot of capillary pipes in concrete. After water evaporates, capillary tension is generated in the capillary, which makes the concrete shrink and deform. Increasing the pore size of capillary can reduce the surface tension of capillary, but it will reduce the strength of concrete. This theory of surface tension was recognized internationally as early as 1960s.
(2) The water-cement ratio is an important factor affecting the shrinkage of concrete, and the water consumption of concrete can be reduced by 25% by using water reducing and crack preventing agent.
(3) Cement dosage is also an important factor of concrete shrinkage. The concrete mixed with water anti-cracking agent can reduce the cement consumption by 65,438 0.5% while maintaining the concrete strength, and its volume can be supplemented by increasing the aggregate consumption.
(4) Water reducing and anti-cracking agent can improve the consistency of cement slurry, reduce concrete bleeding and reduce shrinkage and deformation. (5) Improve the cohesive force between cement slurry and aggregate, and improve the crack resistance of concrete.
(6) Concrete is restrained to produce tensile stress when it shrinks, and cracks will occur when the tensile stress is greater than the tensile strength of concrete. The water reducing and anti-cracking agent can effectively improve the tensile strength of concrete and greatly improve the anti-cracking performance of concrete.
(7) Adding admixture can make concrete compact, effectively improve the carbonation resistance of concrete and reduce carbonation shrinkage.
(8) The retarding time of concrete mixed with water reducing and anti-cracking agent is appropriate, which can effectively prevent the rapid hydration and heat release of cement and avoid the increase of plastic shrinkage caused by long-term non-coagulation of cement.
(9) The concrete mixed with admixture has good workability, and the surface is easy to level off, forming a micro-membrane, reducing water evaporation and drying shrinkage.
Many additives have the functions of retarding setting, increasing workability and improving plasticity. In engineering practice, we should do more experimental comparison and research in this field, which may be simpler and more economical than simply improving external conditions.
Practice has proved that the common cracks in concrete are mostly surface cracks with different depths, mainly due to the sudden drop of temperature in cold areas caused by temperature gradient, which is easy to form cracks. Therefore, the thermal insulation of concrete is particularly important to prevent early cracks on the surface.
From the point of view of temperature stress, thermal insulation shall meet the following requirements:
(1) Prevent the temperature difference between the inside and outside of concrete and the concrete surface gradient, and prevent surface cracks.
(2) In order to prevent concrete from freezing, the lowest temperature of concrete during construction should be no lower than the stable temperature of concrete during its service life.
(3) Prevent the old concrete from supercooling, so as to reduce the constraint between the new and old concrete.
The main purpose of early curing of concrete is to maintain suitable temperature and humidity conditions, so as to achieve two effects. On the one hand, it can protect concrete from adverse temperature and humidity deformation and prevent harmful cold shrinkage and dry shrinkage. On the one hand, the hydration of cement goes smoothly to achieve the designed strength and crack resistance.
Appropriate temperature and humidity conditions are interrelated. Thermal insulation measures often have a moisturizing effect on blood coagulation.
Theoretically, the water content of fresh concrete can completely meet the requirements of cement hydration. However, due to evaporation and other reasons, it often causes water loss, thus delaying or preventing the hydration of cement, and the surface concrete is most easily and directly affected by this adverse effect. Therefore, the first few days after concrete pouring is the key period of maintenance, and more attention should be paid in construction.
In a word, there are different theories about the causes and calculation methods of concrete cracks, but by observing specific prevention and improvement measures, analyzing and summarizing the problems in concrete construction, and combining various prevention and treatment measures, concrete cracks can be completely avoided. Discussion on construction technology of concrete crack treatment: This paper analyzes the causes of concrete cracks. In order to ensure the effect and durability of cast-in-place concrete, besides the correct and reasonable design, the construction technology level must be strictly improved. The appearance of cracks will not only affect the integrity and stiffness of the structure, but also cause strong corrosion of steel bars, accelerate the carbonation of concrete, and reduce the durability, fatigue resistance and impermeability of concrete. Therefore, according to the nature and specific conditions of cracks, we should treat them differently and deal with them in time. Ensure the safe use of the building.
Keywords: concrete; Cast in place; Cracks; quality
Concrete, referred to as "concrete" for short, refers to the general name of engineering composite materials in which aggregates are bonded into a whole by cementing materials. Generally speaking, the word concrete refers to cement concrete, which is made of cement as cementing material, sand and stone as aggregate, mixed with water (with or without additives and admixtures) in a certain proportion, stirred, shaped and cured, also known as ordinary concrete. It is widely used in civil engineering. According to the use function, it mainly includes: structural concrete, thermal insulation concrete, decorative concrete, waterproof concrete, fire-resistant concrete, hydraulic concrete, marine concrete, road concrete, radiation-proof concrete and so on. According to the construction technology, mainly include. Centrifugal concrete, vacuum concrete, grouting concrete, shotcrete, roller compacted concrete, extruded concrete, pumping concrete, etc. According to the reinforcement methods, there are plain concrete, reinforced concrete, steel wire mesh cement, fiber concrete and prestressed concrete. According to the workability of concrete mixture; Hard concrete, semi-hard concrete, plastic concrete, flowing concrete, high fluidity concrete, flowing concrete, etc.
1 Causes of concrete cracks
There are many reasons for concrete cracks, but the fundamental reason is that the tensile stress in concrete exceeds the tensile strength of concrete. Specifically, it can be attributed to three reasons: temperature and humidity changes, excessive deformation caused by external loads, and improper construction methods. The specific types are:
(1) Cracks caused by cement shrinkage. This kind of crack appears on the concrete surface, which is relatively small. Cement is a kind of hydraulic material with drying shrinkage. In the early stage of hardening, cracks may occur if the curing is improper and the water is insufficient.
(2) Temperature difference changes, cracks caused by thermal expansion and contraction. This kind of crack generally appears in the environment with large temperature difference and in the member or structure with large area or length, and no expansion joint is left at the appropriate position.
(3) Cracks caused by stress concentration. This kind of crack usually appears at the corner of yin and yang or the bearing of concrete slab. It is caused by insufficient reinforcement of negative bending moment steel bars in the plate or excessive spacing between thick steel bars.
(4) Improper use leads to overload and excessive deformation leads to cracks. This kind of crack usually appears in the tensile area of concrete flexural members.
(5) Cracks caused by tension. In the process of tensioning prestressed reinforced concrete members, cracks may occur if they are not well controlled. This kind of crack usually appears at the end of the prestressed member or the corner of the upper surface of the plate.
(6) Cracks caused by uneven settlement. Due to the uneven settlement of the foundation, the foundation, ring beam, girder and other components are stretched too much and cracks appear.
(7) In the construction process, cracks will occur in the structure due to the vibration, deformation or displacement of the formwork in the initial setting stage of concrete.
(8) Cracks caused by premature loading. During the construction, due to premature formwork removal, the concrete strength did not meet the design requirements, and the load was added in advance. Overloading parts and causing cracks.
(9) If the construction joint is not handled properly, cracks may appear in the construction joint.
(10) During demoulding, transportation, stacking and hoisting of precast concrete members, the compression zone is in tension for various reasons, which may lead to cracks in the members.
2 the harm of concrete cracks
Concrete is a kind of multi-component composite material, under the condition of temperature and humidity change. Hardening and volume deformation. Due to the inconsistent deformation of various materials, mutual restraint produces initial stress, which leads to invisible microcracks between aggregate and cement bonding surface or cement itself, which are generally called microcracks. The distribution of these microcracks is irregular and discontinuous. However, under the load or under the condition that the temperature changes further and the maintenance is not in place, the cracks begin to expand and gradually connect with each other, resulting in larger cracks visible to the naked eye, which become macro cracks, and serious cracks run through the floor and become harmful cracks. This kind of crack will cause serious harm to the bearing capacity, fire resistance, impermeability, steel corrosion resistance and chemical corrosion resistance of the structure.
Chemical erosion, freeze-thaw cycle, carbonization, steel corrosion, alkali-aggregate reaction, etc. , will have a destructive effect on concrete structures. The occurrence or progress of these failures is not only affected by the material properties of concrete itself, but also by cracks. Generally, it takes 2 ~ 3 months from form removal to decoration completion, and some large-scale projects have to be constructed across the year. At this time, CO2, SO2 and rainwater in the air will enter the concrete along the cracks, which will accelerate the corrosion of steel bars and the reaction and carbonization of alkali aggregate. Thus leading to the deterioration of durability and shortening the service life of the building.
3 concrete crack treatment measures
The repair measures of concrete cracks mainly include the following methods: surface repair, grouting, caulking and structural reinforcement. Concrete substitution method, electrochemical protection method and bionic self-healing method.
3. 1 surface repair method
Surface repair method is a simple and commonly used repair method, which is mainly suitable for the treatment of surface cracks and deep cracks that are stable and do not affect the bearing capacity of the structure. The usual treatment measures are to smear cement slurry, epoxy mastic or anticorrosive materials such as paint and asphalt on the surface of cracks. In order to prevent concrete from cracking due to various effects, measures such as sticking glass fiber cloth on the crack surface can usually be taken. 3.2 Grouting and Mosaic Sealing Method
Grouting method is mainly suitable for repairing concrete cracks that have influence on structural integrity or have anti-seepage requirements. It uses pressure equipment to press cementing material into concrete cracks, and the cementing material hardens and forms a whole with concrete, thus achieving the purpose of plugging and strengthening. Commonly used cementing materials are cement slurry, epoxy resin, methacrylate, polyurethane and other chemical materials.
Sealing method is the most commonly used method in crack sealing. Usually, a groove is opened along the crack and filled with plastic or rigid water-stopping material to seal the crack. Commonly used plastic materials include PVC mastic, plastic ointment, butyl rubber, etc. The commonly used rigid waterproof material is polymer cement mortar.
3.3 Structural reinforcement methods
When cracks affect the performance of concrete structures, it is necessary to consider the reinforcement of concrete structures. The common methods of structural reinforcement mainly include: increasing the cross-sectional area of concrete structure. Steel is wound around the corner of the member, reinforced by prestress method, bonded steel plate, added fulcrum and sprayed concrete.
3.4 concrete replacement method
Concrete replacement method is an effective method to deal with seriously damaged concrete. This method is to remove the damaged concrete first, and then replace it with new concrete or other materials. Commonly used alternative materials are: ordinary concrete or cement mortar, polymer or modified polymer concrete or mortar.
3.5 Electrochemical protection method
Electrochemical corrosion protection is to change the environmental state of concrete or reinforced concrete through the electrochemical action of external electric field in the medium. Passivate the reinforcement to achieve the purpose of anticorrosion. Cathodic protection, chloride extraction and alkali recovery are three common and effective methods in chemical protection. The advantage of this method is that the protection method is less affected by environmental factors, and it is suitable for long-term corrosion protection of steel bars and concrete, which can be used for both cracked structures and new structures.
3.6 bionic self-healing method
Bionic self-healing method is a new type of crack treatment method, which imitates the function of biological tissue and automatically secretes some substances to the injured part to heal the injured part. Adding some special components (such as liquid core fiber or capsules containing binder) to the traditional components of concrete, an intelligent bionic self-healing neural network system is formed in concrete. When cracks appear in concrete, some liquid core fibers will be secreted to make the cracks heal again.